In a novel I read, the Romulan War series, a certain tactic was used involving a Romulan bird of Prey ramming into a planet at warp velocity to change a class M planet into some other class through the release of all that energy into the planet. In the book the crew of the Romulan ship sacrificed their lives to destroy an M class planet in this way. But who's to say a starship employing such a tactic needs to be manned? One can develop warp missles that are launched from a planet or starship designed to impact with another planet at its maximum warp speed. But I really think this shouldn't be allowed, because once this cat is out of the bag it would be employed endlessly.

Instead I would have a rule that if a warp bubble intersects a mass that is significantly greater than the starship that's generating it, the warp bubble should simply collapse and the starship stop. The ship simply resumes the velocity it had relative to the planet before it entered warp speed, conservation of momentum holding, so the ship would simply crash into the planet at a typical low sublight speed and perhaps make a crater in the planet's surface, that's it.